So I don't have to change the sites manually I check the date. Here is a snippet of php code to check the date and only include your .js on Jan 18, 2012:

<? if(date("Ymd")=="20120118") echo '<script type="text/javascript" src="hxxp//js.sopablackout.org/sopablackout.js"></script>'; ?>

Note: replace hxxp with http.